Mostly based on determiningthe time elapsed betweenknown historical events—i.e. the Flood, Abraham,etc. by summing lists ofgenerations and the reignsof various rulers. Assumedroughly 3 generations percentury and about 25years/ruler typically

History (cont.)

Kepler’s

Chronology

Based on the belief that Earthwas created at the summersolstice when the solarapogee was at the head ofthe constellation Aries.Using the known rate atwhich the solar apogeemoved, a date could becalculated.

We know (now!) that most sealevel declines locally are dueto land UPLIFT. There areother places where the landlevel is falling. These effectsare due to geological forcesnot understood in the day ofDeMaillet. The moderntheories of geology were overa century away, and almost250 years before platetectonics.

Most estimates in the pre-radioactive scientific era relied ontheoretical models that were incomplete and had too littlereliable measurements as inputs.

The consensus ages collected around predictions of a fewhundredMyr—over 10 times too small!

What was needed was a “clock” that was unaffected byordinary physical processes, including geological, solar,mechanical, etc. events that could affect chemical reactions,weather patterns, erosion and sedimentation that couldchange age estimates dramatically.

The Radioactive Era

Radioactivity was discovered by Henri Becquerel in 1896 inuranium salts.

Radioactivity was thought to be like X-rays, a form ofelectromagnetic radiation initially.

All radioactive elements have average lifetimes, but often wespeak of half-lives. A half-life is defined as the amount oftime it takes for a radioactive material to decay to ½ of itsoriginal amount, i.e. when we have:

This implies that the half-life is:

021NN2ln212/1/2/1tetAnalyzing the Rubidium-StrontiumClock

Rb-87 decays to Sr-87 in ahalf-life of 48.8 billionyears.

A long-lived radioactiveelement is needed to datesomething that is very old.

Carbon dating is only goodfor relatively short timeperiods, about 70,000 yrs.